A computed-tomography (CT) apparatus including a CT scanner including a rotating X-ray source, and a plurality of photon-counting detectors (PCDs) arranged in a fixed detector ring to capture incident X-ray photons emitted from the X-ray source. The plurality of PCDs includes a first plurality of first PCDs, each first PCD having a first collimator on a surface of the first PCD to block X-ray photons emitted from the X-ray source, the first collimator having openings of a first size, and a second plurality of second PCDs, each second PCD having a second collimator on a surface of the second PCD to block the X-ray photons emitted from the X-ray source, the second collimator having openings of a second size, the first size being different from the second size.
